CRITICAL EVALUATION OF THERMODYNAMIC PROPERTIES AND GEOCHEMICAL RELATIONSHIPS FOR SELECTED ARSENIC SPECIES

Thermodynamic data are essential as input to models that attempt to interpret the geochemistry of environmentally important elements such as arsenic. Unfortunately, the data for mineral phases of arsenic and their aqueous solubilities have been inadequately evaluated. This paper presents the results of a simultaneous, weighted, least-squares multiple regression on 77 thermochemical measurements of elemental arsenic, arsenic oxides, arsenic sulfides, and their aqueous hydrolysis products. These data were selected from a search of several hundred papers. To test the compatibility of the thermodynamic data with observations in nature, the resultant values for free energy, enthalpy, entropy, and heat capacity are used to derive mineral stability relationships for native arsenic, claudetite (As₂O₃), arsenolite (As₂O₃), orpiment (As₂S₃), and realgar (AsS) using pe-pH diagrams and known occurrences and transformations in the environment. Claudetite is the stable arsenic trioxide phase under standard state conditions and transforms to arsenolite below about –33^oC. Both claudetite and arsenolite form as weathering products of arsenopyrite, realgar, and native arsenic but arsenolite is the metastable phase at earth’s surface conditions. At constant P_S2, realgar and native arsenic become increasingly more stable than orpiment with increasing temperature. Realgar has two polymorphs, pararealgar and alacrinite, but thermodynamic data are not available for them. A new pe-pH diagram shows the relative stabilities of As₂O₅, claudetite, orpiment, realgar and arsenic in the As-S-O system at 298.15 K and 1 atm. Native arsenic is only stable under very strong reducing conditions but is more stable at higher temperature. This critical evaluation of published data for selected arsenic species provides much more consistent data for geochemical modelling and the interpretation of geochemical processes involving arsenic in the environment.